Instructors Guide: Atoms and Their Isotopes

Transcription

1 Instructors Guide: Atoms and Their Isotopes Standards Connections Connections to NSTA Standards for Science Teacher Preparation C.3.a.1 Fundamental structures of atoms and molecules. C.3.b.27 Applications of chemistry C.3.b.30 Physics, including motions & forces C.1.2 Nature of scientific evidence and use of models for explanation. Connections to the National Science Education Standards: Structure of atoms: Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together. The atom's nucleus is composed of protons and neutrons, which are much more massive than electrons. When an element has atoms that differ in the number of neutrons, these atoms are called different isotopes of the element. Radioactive isotopes are unstable and undergo spontaneous nuclear reactions, emitting particles and/or wavelike radiation.. Connections to New York Chemistry Core Curriculum: 3.1b 3.1 g, m Use models to describe the structure of an atom. (3.1b,3.1c,3.1d) Determine the number of protons or electrons in an atom or ion when given one of these values. (3.1e) Calculate the mass of an atom, the number of neutrons or the number of protons, given the other two values. (3.1f) Interpret and write isotopic notation. (3.1g) POGIL 2005, /8

2 Atoms of an element that contain the same number of protons but a different number of neutrons are called isotopes of that element. (3.1m) Hints for Facilitation Exercises: A Limiting response to a student question might be Atoms are neutral because the number of protons equals the number of electrons. A Promoting response to a student question might be To be neutral, the positive and negative charge must balance by canceling out. Which subatomic particles have positive & negative charges that must cancel out? Questions in the activity that may cause difficulty for students: Problem 1a: Students may have difficulty with exponents. Students can be directed to use their calculators, and to review scientific notation and the rules for division, multiplication, addition, and subtraction operations in scientific notation. Problem 1b: Accept various answers that have the same magnitude of powers of tens as the ratio of the atom s (radius) diameter to the nucleus diameter (radius). Problem 1c: Student should review the volume of a sphere, and assume that atoms have spherical shapes. V = π r 3 Suggestions for appropriate demonstrations, manipulatives, etc., that will enhance the activity. Demonstration/ Manipulatives: Students can draw Bohr models of the isotopes of atoms used in the POGIL model or the activity questions. Students can construct models of atoms and their isotopes using fruit loops and glue sticks. (This can be done prior to/or after the POGIL activity). To construct fruit loop models of isotopes (atoms), students can use different colors of fruit loops to represent protons, neutrons & electrons, & glue these unto construction paper to form Bohrs model of atoms. POGIL 2005, /8

3 Questions to ask about modeling activity: 1. How are isotopes (of the same element) different from each other? How are they alike? (They have same atomic #, same # of protons, but a different mass #, different # of neutrons). 2. Are isotopes different elements? Explain. (No, the atomic number identifies the element). 3. Do isotopes need to be neutral? (No, they may contain a different # of electrons.). 4. What is the importance of neutrons to the structure of atoms? (What is the purpose of neutrons in the nucleus of an atom)? (They are needed to maintain the stability of the nucleus, by minimizing proton-proton repulsion). 5. Do isotopes of the same element have all the same chemical properties? Explain.(Yes, the outermost (valence shell) electrons determine the chemical properties of atoms). 6. Do isotopes of the same element have all the same physical properties? Explain. (No, not all. Those properties that are mass dependent are different. This is the basis of their separation by a mass spectrometer). 7. Give an example of a different physical property of an isotope? Explain. (Diffusion, higher mass isotopes travel slower than lower mass isotopes). 8. Name one practical application of isotopes. (Radioisotope & others used as tracers, radioisotope use in medicine diagnosis, food preservation. Chemical synthesis using heavy isotopes, nuclear enrichment of desired isotope for use in power plants/ weapons). POGIL 2005, /8

4 Why? Atoms and Their Isotopes Atoms and isotopes are identified by the numbers of protons, neutrons and electrons that they contain. Before you can understand the properties of atoms, how atoms combine to form molecules, and the properties of molecules, you must be familiar with the number of protons, neutrons and electrons associated with atoms. Success Criteria Identify the composition of atoms and their isotopes in terms of the numbers of protons, neutrons, and electrons. Use atomic symbols to represent different atoms and their isotopes. Efficient use of Periodic Table as a source of data. Resources Periodic Table Information From the perspective of a chemist, the entire world is composed of atoms, and atoms are composed of protons, neutrons and electrons. Protons and neutrons are about 2000 times heavier than an electron. A proton has a charge of +1, a neutron has no charge and an electron has a charge of -1. The nucleus is very dense and very small compared to the entire atom. The properties of atoms are determined by the numbers of protons, neutrons and electrons that they contain. Atoms with the same number of protons but different number of neutrons are called isotopes of an element. The isotopic notation for an atom includes the following information: symbol of the element, the element's atomic number (Z) which specifies the number of protons in the nucleus, and the mass number (A) which indicates the number of protons plus neutrons in the nucleus. [The number of electrons in a neutral atom is equal to the number of protons in the nucleus of the atom. The mass contributed by the electrons in an atom is very small, so it is not included when calculating the mass number.] A X Z Atomic Symbol Notation Mass Number Atomic Symbol Atomic Number POGIL 2005, /8

6 Key Questions 1. What information is provided by the atomic number, Z? Number of protons or nuclear charge. In neutral atom also number of electrons. 2. What information is provided by the mass number, A? Sum of the number of protons and neutrons located in the nucleus of an atom. To get the neutrons, subtract atomic # from mass #. 3. What is the relationship between the number of protons and the number of electrons in an atom? In a neutral atom the number of protons equals the number of electrons. 4. Because of the relationship between the number of protons and number of electrons in an atom, what is the electrical charge of an atom? No charge, the charge on the atom is zero. The atom is neutral. 5. Where are the protons and neutrons located in an atom? In the nucleus, the dense center of an atom. 6. What do the two sodium isotopes shown in the model have in common with each other? They have the same number of protons and the same atomic number. They also have the same number of electrons. 7. How do the two sodium isotopes shown in the model differ from each other? They have a different number of neutrons, and a different mass number. 8. What distinguishes an atom of one element from an atom of another element? The number of protons in the nucleus, the atomic number. POGIL 2005, /8

7 Exercises Describe the similarities between 17 Cl, and 17 Cl. They have the same number of protons (and electrons), and the same atomic number Describe the differences between 17 Cl, and 17 Cl. The have a different number of neutrons, and a different mass number. 3. Write the atomic symbols for two isotopes of carbon, C, one with 6 neutrons and the other with 7 neutrons C and 13 6C 4. Use a periodic table to fill in the missing information in the following table. Name Symbol Atomic Number Z Mass Number A Number of Neutrons Number of Electrons oxygen nitrogen sulfur 16 8O N S hydrogen hydrogen magnesium 1 1 H H Mg magnesium Mg uranium krypton U Kr POGIL 2005, /8

8 Problems 1. The radius of a Cl nucleus is 4.0 fm, and the radius of a Cl atom is 100 pm. (1 fm = 1 x m; 1 pm = 1 x m).how many times larger is the diameter of the Chlorine atom than the diameter of the Chlorine nucleus? = About 30,000 times larger. 2. Identify two objects that have this same ratio of lengths. 25 kilometer field length, to 1 meter stick; 25 meter auditorium to 1 millimeter insect. Possibly the radii of other atoms compared to the radii to their nuclei (ex. S). 3. How many times larger is the volume of the atom than the volume of the nucleus? 3 Vol sphere πr 10 3 = ( 1 10 ) 15 3 ( 4 10 ) 30 ( 1 10 ) 44 ( ) = = = The volume of the atom is 2 x its nucleus. 13 = times larger than POGIL 2005, /8

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